Enable Latest Material Design 2.0 on Message App

Pre-Requisites

1.Rooted Phone
2.A file explorer with root option.

Let's Do it

1. Install Any file explorer that supports root ex. Amaze (it's free). This will be required for modifying the shared_prefs settings stored in the data partition.
2. Open the file explorer and go to the root section. Once in root go to /data/data/com.google.android.apps.messaging/shared_prefs/. In this folder open PhenotypePrefs.xml. When the popup opens to select editor.
3. In the top right corner, tap the three dots and select find, search for bugle__phenotype_enable_m2.
4. The value should be set to false at the end. Set this to true.
5. Go back to the find option and find the option bugle_phenotype_enable_phenotype_override. You are also going to want to set this true.
6. Once you do this, force close the Messages app.

Once you re-open the app you will see the Material Design 2.

The only problem with doing this is that this redesign will only last once till you force close or clear it. If you close the app out of Recents, it will be reset to the original design.